Surface treatment for electroplating

ABSTRACT

A method for improving the adhesion between a metal coating and a nylon surface. The method includes the steps of treating a nylon surface with an aqueous solution of iodine and iodine ion in which the concentration of the iodine is at least about 0.2 molar and the concentration of the iodine ion is at least about 0.3 molar, and subsequently washing the surface to remove the iodine as quickly as possible prior to plating the polymer.

United States Patent Inventor lsmat A. Abu-lsa Birmingham, Mich.

Appl. No. 867,006

Filed Oct. 16, 1969 Patented Sept. 21, 1971 Assignee General Motors Corporation Detroit, Mich.

SURFACE TREATMENT FOR ELECTROPLATING 8 Claims, 2 Drawing Figs.

u.s. Cl 117/47,

117/138.8,117/213, 8/115.5 Int. Cl B44d l/02 Field of Search 1 17/47 R,

[56] References Cited UNITED STATES PATENTS 2,251,508 8/1941 Watson l17/l38.8 (X) 3,075,823 l/1963 Reyerson et a1. ll7/138.8 (X) Primary ExaminerRalph S. Kendall Assistant Examiner-Caleb Weston Attorneys-William S. Pettigrew, R. J. Wallace and Lawrence B. Plant ABSTRACT: A method for improving the adhesion between a metal coating and a nylon surface. The method includes the steps of treating a nylon surface with an aqueous solution of iodine and iodine ion in which the concentration of the iodine is at least about 0.2 molar and the concentration of the iodine ion is at least about 0.3 molar, and subsequently washing the surface to remove the iodine as quickly as possible prior to plating the polymer.

PATENTED SEPZI l9?! 7,607,353

INVENTOR ATTORNEY SURFACE TREATMENT FOR lELECTROPLATING Conventional processes for plating plastics involve treating the plastic with a strongly acidic oxidizing solution to deglaze or etch the surface and render it hydrophilic. After etching, a reducing agent, known as a sensitizer, is absorbed on the surface. Typical known sensitizers are tin and titanium salts such as stannous chloride. Following sensitizing the surface is activated by treating it with an aqueous solution of a salt of a catalytic or precious metal, such as gold, silver or the palladium group metals, with palladium being the most popular. The sensitized surface reduces the catalyst-bearing salt to the corresponding catalytic metal and thereby provides a plurality of discrete nucleation centers on the surface to be plated. Electroless coatings of copper or nickel are subsequently deposited followed by any desired sequence of electrodeposited coatings. A typical electroless copper bath contains copper salts, copper complexing agents, a pH buffer and a reducing agent. A typical electroless nickel bath includes a nickel salt and reducing agents such as sodium hypophosphite, acetic acid and sodium hydroxide. These baths are essentially autocatalytic in that after initiation by the catalytic metal the copper or nickel plate becomes itself a catalyst and plating continues.

It is an object of this invention to provide improved adhesion between nylon substrates and metal deposits thereon. A further object of this invention is to accomplish the first object in a manner which eliminates the heretofore required steps of etching the surface with oxidizers and acids and subsequently specifically sensitizing the surface.

As it relates to the plating of nylon, this invention differs from conventional processes by including the steps of treating the surface with an aqueous solution of iodine and iodide ion and subsequently washing the surface to remove any absorbed iodine. Improved adhesion results between the nylon and subsequently deposited metal layers without the need for surface deglazing or etching heretofore believed essential. I have further found that I can improve the speed of the iodine wash and at the same time eliminate the conventional sensitization step by including in the wash solution a reducing agent which reduces the iodine to iodide ion and remains absorbed on the surface to further reduce the catalytic salts.

FIG. I is a 2700 magnification photomicrograph of a Nylon-6 surface before the iodine/iodide treatment of this invention.

FIG. 2 is a 2700 magnification photomicrograph of a Nylon-6 surface after the iodine/iodide treatment of this invention.

I have found that plated metals adhere best to nylon which has first been treated with an aqueous solution of iodine and iodide ion in which the concentration of the iodine is in excess of about 0.2 molar and the concentration of the iodide ion is in excess of about 0.3 molar and subsequently washed to remove the iodine from the surface. For electroless nickel and electrolytic copper coated nylon samples aged for 48 hours, In stron peel tests registered adhesion values averaging about 16 lbs/inch width when 1 inch wide samples were peeled at a rate of 1 inch per minute at 90 from the surfaces.

I have found that aqueous solutions containing iodine in the concentration range of about 0.2 molar to saturation (about 1.0 molar) and iodide ion in the range of 0.3 molar to satura tion (about 1.5 molar) are most effective for treating the nylon. I prefer to add the iodide ion by dissolution of its alkali metal' salt, e.g., potassium iodide since solutions thereof are substantially neutral. These solutions are effectively operated at temperatures ranging from about room temperature to about 70 C. and, depending on temperature and concentration, the treatment time will vary from about seconds to about 10 minutes. Tests have shown that when the iodine concentration is less than about 0.2 molar the surface conversion is either too slow for practical applications or a nonuniform metal deposit is formed. If the iodide concentration is below about 0.3 molar, not enough iodine can be put into solution to be effective.

A particularly preferred form of this invention comprises treating the nylon for about one-half minute with a 40 C. aqueous solution containing about 0.4 molar iodine, 0.6 molar KI.

While the precise nature of the iodine attack on the Nylon is not known, it is believed that the iodine causes crystal transformation of the Nylon at the surface such that the normal alpha form is converted to the gamma form. Tests have indicated that there is neither a loss of material nor an oxidation reaction occurring on the surface. FIG. 1 depicts a Nylon-6 surface before treatment in accordance with this invention. FIG. 2 depicts the same nylon after treatment and shows the roughened surface characterized by a plurality of voids or cavities (dark) formed in the surface by the iodine attack and left after the iodine removal.

Prior to subsequent plating operations the iodine is washed from the surface. This is done as rapidly as possible to insure that only the surface is attacked. As long as there is some iodine present it will continue to penetrate deeper into the nylon which should be avoided. Further, any iodine carried into the subsequent steps will contaminate the operation and poison catalysts used. It has been observed that if a washing solution cannot remove the iodine in less than about 15 minutes that solution is ineffective for purposes of this invention. Preferably complete iodine removal should be accomplished within the first 10 minutes of washing. A simple water wash will not remove the iodine fast enough since iodine has a very low solubility in water. Additionally, since the iodine has already penetrated the nylons surface, a washing solution must be used which will likewise penetrate the nylon surface, seek out and dissolve the iodine before it can penetrate even deeper.

In accordance with another aspect of my invention, I have found two types of wash solutions which are particularly effective for rapidly removing the iodine from the treated nylon. The first type includes aqueous solutions of iodine solvents such as acetone, ethylene glycol or the like. Most of the glycerol family of compounds would be acceptable as iodine solvents for this application. Ethylene glycol solutions containing about 5 percent by volume water have been found to be particularly effective. The second type of washing solution is an aqueous solution of readily oxidlizable compounds such as sodium thiosulfate, stannous chloride, or hydrazine. Upon reaction with these materials, the iodine is reduced to the more soluble iodide ion. In actual practice I prefer a mixture of the two types of washing solutions and have had significant success with a washing solution comprising about 5 percent by volume water, about 3 g./l. sodium thiosulfate and the balance ethylene glycol. Such washing solutions have demonstrated the ability to remove iodine in less than about 4 minutes.

Another significant aspect of my invention revolves around the elimination of the conventional sensitization step. I have found that by adding a reducing agent to the iodine washing solution in amounts in excess of that required to reduce the iodine, the sensitization step can be eliminated and after washing and rinsing the nylon directly activated by treatment with a conventional catalyst-bearing activation solution. Accordingly, washing solutions containing reducing agents in excess of that required to reduce the iodine provide the twofold benefit of accelerating iodine removal and eliminating the need for a separate sensitization step.

After the iodine and wash treatments conventional plating techniques are employed to provide the desired finished product. In this regard then, and when sensitization is necessary, the nylon would be sensitized by treatment with a sensitizer, such as a solution containing about 50 g./l. of stannous chloride in a 0.5 normal sodium hydroxide solution. Unlike conventional acid sensitizing solutions for other polymers, sodium hydroxide solutions of stannous chloride reduces the number of nucleation sites formed during the activation step. Such alkaline solutions will vary from about 20 g./l. to about g./ ll. stannous chloride. It was discovered that when a conventional acid sensitizer solution is used so many nucleating sites are formed that there is virtually spontaneous breakdown or fallout of the palladium from the palladium chloride bath. An alternative to the alkaline stannous chloride sensitization is the use of a slightly acid (1-2 drops/liter HCl), very dilute (i.e., of the order of l g./l.) stannous chloride solution.

Following sensitization and rinsing, the surface would next be treated with any art-known activating solution such as one containing about 0.25 grams of palladium chloride and about 4.6 ml. of hydrochloric acid per liter of solution. Activation is conveniently carried out at room temperature. Following activation and rinsing, an electroless metal coating would be deposited and subsequent electrodeposits are placed thereover in any desired art-known fashion. I prefer an electroless nickel deposit directly onto the activated nylon. A typical electroless deposition bath for this purpose contains about 16 grams of nickel sulfate hexahydrate, about 14 grams of sodium hypophosphite, about 12.7 ml. of acetic acid and about 6.8 grams of sodium hydroxide in one liter of water. Such a bath is conveniently operated at about 78 C. After the electroless metal layer has been deposited, any desired sequence of electrodeposits may be utilized to finish plate the nylon.

A particularly successful plating sequence demonstration excellent adhesion and durability after repeated testing was one in which a Nylon-6 sample was coated with an electroless nickel layer of about 0.01 mils thick followed by a Udylite U- BAC copper layer 0.8 mils thick. A Udylite N2E semibright nickel layer 0.5 mils thick was deposited atop the copper followed by a Udylite 66 bright nickel layer 0.3 mils thick. A conventional chromium layer 0.003 mils thick finished the part. The Nylon-6 had been treated for rminute with a 40 C. solution containing 0.4 M iodine and 0.6 M iodide ion. The sample was thermal cycled as follows:

(a) 180 F. for 1 hour; (b) room temperature for minutes; (c) F. for 1 hour; and (d) room temperature for 15 minutes. This thermal sequence was repeated four times and the plated nylon checked for cracks or blisters. The absence of observable cracks or blisters was indicative of the excellent adhesion obtained by this process.

This process has been found to be acceptable for the several commercially available nylons such as Nylon-4, Nylon-6, Nylon-66, Nylon-68, Nylon-610, Nylon-7, Nylon-8, Nylon-9 and Nylon-l 1.

While I have described my invention solely in terms of certain specific embodiments thereof, I do not intend to be limited thereto except to the extent hereinafter set forth.

I claim:

1. A method of improving the adhesion between the surface of a nylon substrate and a layer of metal electrolessly deposited thereon comprising the steps of treating said surface with an aqueous solution of iodine and iodide ion in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar and washing said surface to remove the iodine during the first 15 minutes of washing and prior to depositing said metal layer.

2. A method of metal plating nylon substrates comprising the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ions in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface with an aqueous solution of an iodine solvent to remove the iodine during the first 15 minutes of washing, treating said surface with a reducing agent to sensitize said surface, treating the sensitized surface with an aqueous solution of salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface.

3. A method according to claim 2 wherein sensitization of said surface is accomplished by treating said surface with an alkaline solution of stannous chloride.

4. A method of metal plating nylon substrates consisting essentially of the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ion in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface free of iodine by treating it in an aqueous solution of an iodine solvent and a reducing agent wherein the concentration of said reducing agent is stoichiometrically greater than is required to reduce the iodine absorbed in said'surface to iodide ion, treating the washed surface with an aqueous solution of a salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface.

5. A method according to claim 4 wherein the concentration of said iodine is about 0.4 molar and the concentration of said iodide in is about 0.6 molar.

6. A method according to claim 4 wherein said washing is accomplished in less than about 10 minutes and said washing solution comprises an aqueous solution of ethylene glycol and a reducing agent selected from the group consisting of sodium thiosulfate, stannous chloride and hydrazine.

7. A method according to claim 6 wherein the concentration of said iodine is about 0.4 molar and the concentration of said iodide is about 0.6 molar, said wash solution comprises an aqueous solution of ethylene glycol and sodium thiosulfate and said washing is accomplished in about 4 minutes.

8. A method of metal plating nylon substrates consisting essentially of the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ion in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface free of iodine by treating it for no more than about 15 minutes in an aqueous solution of a reducing agent in which the concentration of said reducing agent is stoichiometrically greater than is required to reduce the iodine absorbed in said surface to iodide ion, treating the washed surface with an aqueous solution of a salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface. 

2. A method of metal plating nylon substrates comprising the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ions in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface with an aqueous solution of an iodine solvent to remove the iodine during the first 15 minutes of washing, treating said surface with a reducing agent to sensitize said surface, treating the sensitized surface with an aqueous solution of salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface.
 3. A method according to claim 2 wherein sensitization of said surface is accomplished by treating said surface with an alkaline solution of stannous chloride.
 4. A method of metal plating nylon substrates consisting essentially of the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ion in which the concentration of said iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface free of iodine by treating it in an aqueous solution of an iodine solvent and a reducing agent wherein the concentration of said reducing agent is stoichiometrically greater than is required to reduce the iodine absorbed in said surface to iodide ion, treating the washed surface with an aqueous solution of a salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface.
 5. A method according to claim 4 wherein the concentration of said iodine is about 0.4 molar and the concentration of said iodide in is about 0.6 molar.
 6. A method according to claim 4 wherein said washing is accomplished in less than about 10 minutes and said washing solution comprises an aqueous solution of ethylene glycol and a reducing agent selected from the group consisting of sodium thiosulfate, stannous chloride and hydrazine.
 7. A method according to claim 6 wherein the concentration of said iodine is about 0.4 molar and the concentration of said iodide is about 0.6 molar, said wash solution comprises an aqueous solution of ethylene glycol and sodium thiosulfate and said washing is accomplished in about 4 minutes.
 8. A method of metal plating nylon substrates consisting essentially of the steps of treating a surface of said nylon for at least about 10 seconds with an aqueous solution of iodine and iodide ion in which the concentration of saId iodine is at least about 0.2 molar and the concentration of said iodide ion is at least about 0.3 molar, washing said surface free of iodine by treating it for no more than about 15 minutes in an aqueous solution of a reducing agent in which the concentration of said reducing agent is stoichiometrically greater than is required to reduce the iodine absorbed in said surface to iodide ion, treating the washed surface with an aqueous solution of a salt of a catalytic metal to activate said surface by providing catalytic nucleating centers on said surface and electrolessly depositing a layer of metal on said activated surface. 